The present invention relates to a process for the production of tin salts. It particularly relates to a process for the production of tin salts such as alkali metal stannates by reacting a tin-containing material with an alkaline solution.
As is well known in the art, tin salts and particularly alkali metal stannates, are highly valuable commercial commodities, finding use in electrolytic baths for the high speed electrotinplating of steel strip. Heretofore, however, the prior art has been unable to develop a satisfactory process for the production of these compounds. The direct reaction of tin metal with an alkali metal hydroxide solution has proven to be commercially impractical due to the extremely slow rate of reaction at room temperature, even with vigorous agitation. It is known to add sodium nitrite or nitrate as oxidizing agents to the direct contact reaction mixture which, while improving the oxidation efficiency of the reaction and oxidizing any stannites present (which stannites adversely affect plating quality) adds impurities and may deleteriously affect the reaction vessel and surrounding atmosphere.
Exemplary of such direct contact processes are the processes disclosed in U.S. Pat. Nos. 1,902,735; 2,416,709; 2,434,283; 2,434,284; and 2,570,473. These patents all teach processes wherein a tin-containing ore is smelted at temperatures of approximately 850.degree. C. in the presence of a small quantity of a reducing agent, such as carbon, to convert the tin present in the ore into an acid and alkali soluble form. The resultant smelted mass is thereafter treated with an aqueous sulfuric acid or alkali metal hydroxide solution to produce either stannous sulfate or an alkali metal stannate. U.S. Pat. Nos. 2,416,709 and 2,434,284 also suggest that the treating solution may contain an oxidizing agent, such as hydrogen peroxide or a soluble nitrate or nitrite in order to accelerate the rate of reaction.
It has also been suggested that alkali metal stannates and stannous sulfate may be produced by contacting tin metal with an acid or alkaline solution in the presence of air. See for example, U.S. Pat. Nos. 121,948; 974,463; 2,436,974; and 2,726,929. However, even with the use of air, the rate of reaction is still undesirably slow, and precludes effective commercial use of this method. Moreover, sparging the solution with air leads to an increase in degradation products and other undesirable impurities.
As a consequence of the slow reaction rate and impurity problems associated with the direct contact method, tin salts have conventionally been commercially produced by other processes. U.S. Pat. No. 3,432,256 discloses a two-step method for the preparation of stannous fluoborate wherein a stannic fluoborate solution is first prepared by contacting tin with an aqueous fluoborate solution in the presence of oxygen, and is then reduced to stannous fluoborate by reduction with tin metal in the absence of oxygen.
A further disadvantage of the prior art direct contact type processes arises in their use to prepare tin salts for electroplating operations. These processes have been found to produce a tin salt product containing undesirably high concentrations of impurities. For tin plating applications, the presence of impurities is particularly undesirable, resulting in poor plating quality.
One approach to this problem of impurities, at least with respect to the production of sodium stannate, is disclosed in U.S. Pat. No. 3,394,061. This patent teaches a process for the production of sodium stannate wherein a tin-containing material is first treated with hydrochloric acid to leach and convert the tin values present therein to stannic chloride. The resulting stannic chloride solution is passed through a bed of activated carbon which preferentially absorbs the stannic chloride from the solution. The stannic chloride is then diluted from the activated carbon bed with an aqueous solution of sodium hydroxide, which converts the stannic chloride to sodium stannate.
It has also been suggested by Brinkmann in "Katalytische Wirkungen Von Aktivkohle", Kolloid-Zeitschrift (1951), pages 116 to 129, that activated carbon may have a catalytic effect on the oxidation of metals by atmospheric oxygen.
The prior art has yet to develop, however, an efficient and economical direct contact type process for the production of tin salts. Accordingly, in view of the large commercial importance of tin salts, and particularly alkali metal stannates, there exists a great need in the art for an economical and efficient method for the production of these compounds.